It is well known that electronic color separation film production by means of an image reproducing system is carried out according to the following method. That is, an original mounted on an input drum is helically scanned by an input head to obtain an image signal thereof. The image signal, after undergoing color and gradation correction processes in an image processor, is input to a comparator to be compared with a halftone dot signal being output from a halftone dot generator to produce a recording signal for driving a recording head, by which a so-called screened image is reproduced onto a photosensitive film mounted on an output drum.
The halftone dot generator comprises a screen memory, in which thresholds corresponding to the density values of halftone dot sub-cells in arranged an matrix are stored. Each of the thresholds is output according to the address of the corresponding recording point to said comparator. The comparator makes the recording head expose (or not expose) the photosensitive film when the density value of the image signal is higher than the corresponding threshold, or not expose (or expose) the film when the density value of the image signal is lower than the corresponding threshold.
Since a screened image is to be produced using a plurality of halftone dots, the thresholds stored in the screen memory are periodically read again and again in accordance with the proceeding of the recording process.
In that regard, it is well known that, when each of color separation images Y (Yellow, M (Magenta), C (Cyan) and K (Black) is recorded at the same screen angle, a moire effect is produced, which deteriorates the quality of the synthesized image of the four color separation images. Therefore, in order to avoid the undesirable phenomenon, the screen angles of the color separation images M, C, Y and K are, for example, determined to be at -15, 15, 0 and 45 respectively.
In reading out the thresholds under the condition of a screen angle, there must be input to the screen memory sub-scanning addresses X.sub.A and main scanning addresses Y.sub.A expressed by equations: ##EQU2## wherein x.sub.n is the position data of the sub-scanning direction (pixel number), y.sub.n is the position data of the main scanning direction (pixel number), while R is the ratio between the length of each side of one pixel and that of one halftone sub-cell.
U.S. Pat. No. 4,350,996 discloses a method, by which, based on the equations (A), the terms pertaining to each of the directions y.sub.n and x.sub.n is respectively computed using a multiplier, and the products are summed. However, the multiplier is required to have a large capacity to perform the computation, which is a factor in increasing the manufacturing cost thereof. The above mentioned patent also discloses a method, by which each term of the equations (A) is increased propotionally according to the advance of the main and the sub-scanning positions, and then the terms pertaining to x.sub.x and y.sub.n are summed up.
However this method necessitates a circuit for increasing each of the terms, therefore the circuit must have greater capacity in order to suppress the gross error produced during the computation process.